When closing the DRM device while a vblank is pending, we access
file_priv after it has been free'd, which gives:
Unable to handle kernel NULL pointer dereference at virtual address 00000000
PC is at __list_add+0x5c/0xe8
LR is at send_vblank_event+0x54/0x1f0
[<c02952e8>] (__list_add) from [<c031a7b4>] (send_vblank_event+0x54/0x1f0)
[<c031a760>] (send_vblank_event) from [<c031a9c0>] (drm_send_vblank_event+0x70/0x78)
[<c031a950>] (drm_send_vblank_event) from [<c031a9f8>] (drm_crtc_send_vblank_event+0x30/0x34)
[<c031a9c8>] (drm_crtc_send_vblank_event) from [<c0339ad8>] (vop_isr+0x224/0x28c)
[<c03398b4>] (vop_isr) from [<c0081780>] (handle_irq_event_percpu+0x12c/0x3e4)
This can be triggered somewhat reliably with:
modetest -M rockchip -v -s ...
Add a preclose hook to the driver so that we can discard any pending
vblank events when the device is closed.
Signed-off-by: John Keeping <john@xxxxxxxxxxxx>